The present invention relates to a glass ceramic, which may be used, in particular as a resistor or as a gas-tight ceramic solder in a spark plug, a method of producing the glass ceramic and a spark plug having such a glass ceramic.
German Published Patent Application No. 196 51 454 refers to a spark plug including a terminal stud connected to a center electrode across a resistor in the form of a fused glass seal arranged between the electrode and the terminal stud. This resistor seal may be made of a glass material or a glass ceramic material, which may be provided with a metal phase in the form of a network to increase the electric conductivity. This metal phase may be achieved by a surface metallization of glass powder deposited in a currentless operation and then fused in the spark plug to form a resistor.
German Published Patent Application No. 196 23 989 refers to a spark plug in which the terminal stud is connected to the center electrode by a burn-off resistor and a contact pin is arranged between the burn-off resistor and the center electrode.
An exemplary glass ceramic according to the present invention is believed to be suitable for use as a glass ceramic seal in a spark plug, in which the seal may have either a high or low resistance, depending on its composition. This exemplary glass ceramic may be stable at temperatures up to more than 1000xc2x0 C., so that it may be used as a burn-off resistor in a spark plug.
This exemplary glass ceramic may be suitable for use in spark plugs having a platinum center electrode, which may be heated up to 950xc2x0 C. in the area of the insulator base during operation. This exemplary glass ceramic may tolerate an operating temperature of 900xc2x0 C. for more than 2000 hours.
Another exemplary glass ceramic according to the present invention may be fused from a glass powder or a glass powder mixture at a process temperature of less than approximately 950xc2x0 C., including a temperature between approximately 850xc2x0 C. and 950xc2x0 C., without requiring the use of a protective gas. This exemplary glass ceramic may have high voltage strength up to 20 kV/mm at room temperature or up to 10 kV/mm at 800xc2x0 C., a thermal expansion coefficient of the glass ceramic being adapted to that of aluminum oxide (Al2O3), which may be an insulator material for use in spark plugs. The thermal expansion coefficient may be approximately 6 ppm/K from 100xc2x0 C. to 200xc2x0 C. and approximately 9 ppm/K from 700xc2x0 C. to 800xc2x0 C. On the basis of these properties, an exemplary glass ceramic of the present invention is believed to be suitable for producing a glass ceramic seal as a burn-off resistor in a spark plug having an adjacent insulator made of dense, pure crystalline aluminum oxide.
The high temperature stability and high voltage strength may be achieved because an exemplary glass ceramic according to the present invention may have refractory phases at least partially or at least in some areas, for example, the phases anorthite, wollastonite and titanite.
Through controlled processing of a starting glass to a starting material, an exemplary glass ceramic may be produced according to the present invention in the form of an electrically conducting glass ceramic solder. The exemplary glass solder may be used, for example, to contact a glass ceramic resistor seal having a different composition to a metal, such as a contact pin, a stud or a center electrode in a spark plug.
An exemplary glass ceramic according to the present invention may at least better guarantee thermally stable and gas-tight contacting of the center electrode or the stud in a spark plug by varying its composition.
With regard to high voltage strength, thermal stability and adaptation of the thermal expansion coefficient to the surrounding insulator made of, for example, aluminum oxide, an exemplary glass ceramic according to the present invention may have a composition including a starting mixture containing approximately 43 wt % to 48 wt % SiO2, approximately 16.5 wt % to 18 wt % Al2O3, approximately 6 wt % to 10.5 wt % TiO2, approximately 0.3 wt % to 1.2 wt % Na2O, approximately 0.3 wt % to 1.2 wt % K2O and approximately 24.5 wt % to 28.5 wt % CaO.
Another exemplary glass ceramic according to the present invention, with regard to the development of refractory phases through a controlled temperature treatment of the starting glass, may be obtained by a glass ceramic having a composition including a starting mixture composed of approximately 45 wt % SiO2, approximately 17 wt % Al2O3, approximately 9 wt % TiO2, approximately 0.5 wt % Na2O, approximately 0.5 wt % K2O and approximately 28 wt % CaO.
To adjust electric properties of an exemplary glass ceramic according to the present invention, two different alternatives may be used. First, a metal phase may be developed, which may be embedded in the glass ceramic and which may be a network. Second, a carbon phase may be developed, which may be embedded in the glass ceramic and which may be a network. The carbon phase may be composed of pyrolyzed carbon.
To produce another exemplary glass ceramic according to the present invention having desired crystalline phases, the fusion temperature of the starting material used may be between 850xc2x0 C. and 950xc2x0 C. The heated starting material may then be kept at this temperature for a first period of time, which may be from 5 to 15 minutes, whereupon the refractory phases may be formed, and the starting material may be converted into a glass ceramic. A glass ceramic is understood in this connection to be a material which, unlike a glass, has crystalline phases in at least some areas.
To produce yet another exemplary glass ceramic according to the present invention having a low resistance, for example, for use as a glass ceramic solder, the glass powder used in an exemplary method according to the present invention may be provided with a metallization composed of a metal with high temperature stability or, alternatively, a compound including a glass powder, a binder and a carbon black powder.